Microbiota dictate T cell clonal selection to augment graft-versus-host disease after stem cell transplantation.

Allogeneic T cell expansion is the primary determinant of graft-versus-host disease (GVHD), and current dogma dictates that this is driven by histocompatibility antigen disparities between donor and recipient. This paradigm represents a closed genetic system within which donor T cells interact with peptide-major histocompatibility complexes (MHCs), though clonal interrogation remains challenging due to the sparseness of the T cell repertoire. We developed a Bayesian model using donor and recipient T cell receptor (TCR) frequencies in murine stem cell transplant systems to define limited common expansion of T cell clones across genetically identical donor-recipient pairs. A subset of donor CD4+ T cell clonotypes differentially expanded in identical recipients and were microbiota dependent. Microbiota-specific T cells augmented GVHD lethality and could target microbial antigens presented by gastrointestinal epithelium during an alloreactive response. The microbiota serves as a source of cognate antigens that contribute to clonotypic T cell expansion and the induction of GVHD independent of donor-recipient genetics.

HA-1-targeted T cell receptor (TCR) T cell therapy for recurrent leukemia after hematopoietic stem cell transplantation.

Relapse is the leading cause of death after allogeneic hematopoietic stem cell transplantation (HCT) for leukemia. T cells engineered by gene transfer to express T cell receptors (TCR; TCR-T) specific for hematopoietic-restricted minor histocompatibility (H) antigens may provide a potent selective anti-leukemic effect post-HCT. We conducted a phase I clinical trial employing a novel TCR-T product targeting the minor H antigen HA-1 to treat or consolidate treatment of persistent or recurrent leukemia and myeloid neoplasms. The primary objective was to evaluate the feasibility and safety of administration of HA-1 TCR-T post-HCT. CD8+ and CD4+ T cells expressing the HA-1 TCR and a CD8-co-receptor were successfully manufactured from HA-1 disparate HCT donors. One or more infusions of HA-1 TCR-T following lymphodepleting chemotherapy were administered to nine HCT recipients who had developed disease recurrence post-HCT. TCR-T cells expanded and persisted in vivo after adoptive transfer. No dose-limiting toxicities occurred. Although the study was not designed to assess efficacy, four patients achieved or maintained complete remissions following lymphodepletion and HA-1 TCR-T, with one ongoing at >2 years. Single-cell RNA sequencing of relapsing/progressive leukemia after TCR-T therapy identified upregulated molecules associated with T cell dysfunction or cancer cell survival. HA-1 TCR-T therapy appears feasible and safe and shows preliminary signals of efficacy. This clinical trial is registered at clinicaltrials.gov as NCT03326921.

Conditioning intensity and peritransplant flow cytometric MRD dynamics in adult AML.

In acute myeloid leukemia (AML), measurable residual disease (MRD) before or after allogeneic hematopoietic cell transplantation (HCT) is an established independent indicator of poor outcome. To address how peri-HCT MRD dynamics could refine risk assessment across different conditioning intensities, we analyzed 810 adults transplanted in first or second remission after myeloablative conditioning (MAC; n = 515) or non-MAC (n = 295) who underwent multiparameter flow cytometry-based MRD testing before as well as 20 to 40 days after allografting. Patients without pre- and post-HCT MRD (MRDneg/MRDneg) had the lowest risks of relapse and highest relapse-free survival (RFS) and overall survival (OS). Relative to those patients, outcomes for MRDpos/MRDpos and MRDneg/MRDpos patients were poor regardless of conditioning intensity. Outcomes for MRDpos/MRDneg patients were intermediate. Among 161 patients with MRD before HCT, MRD was cleared more commonly with a MAC (85 of 104; 81.7%) than non-MAC (33 of 57; 57.9%) regimen (P = .002). Although non-MAC regimens were less likely to clear MRD, if they did, the impact on outcome was greater. Thus, there was a significant interaction between conditioning intensity and "MRD conversion" for relapse (P = .020), RFS (P = .002), and OS (P = .001). Similar findings were obtained in the subset of 590 patients receiving HLA-matched allografts. C-statistic values were higher (indicating higher predictive accuracy) for peri-HCT MRD dynamics compared with the isolated use of pre-HCT MRD status or post-HCT MRD status for prediction of relapse, RFS, and OS. Across conditioning intensities, peri-HCT MRD dynamics improve risk assessment over isolated pre- or post-HCT MRD assessments in patients with AML.

Role of chimeric antigen receptor T-cell therapy: bridge to transplantation or stand-alone therapy in pediatric acute lymphoblastic leukemia.

PURPOSE OF REVIEW: To discuss the curative potential for chimeric antigen receptor T-cell (CAR-T) therapy, with or without consolidative hematopoietic stem cell transplantation (HCT) in the treatment of children and young adults with B lineage acute lymphoblastic leukemia (B-ALL). RECENT FINDINGS: CAR-T targeting CD19 can induce durable remissions and prolong life in patients with relapsed/refractory B-ALL. Whether HCT is needed to consolidate remission and cure relapse/refractory B-ALL following a CD19 CAR-T induced remission remains controversial. Preliminary evidence suggests that consolidative HCT following CAR-T in HCT-naïve children improves leukemia-free survival. However, avoiding HCT-related late effects is a desirable goal, so identification of patients at high risk of relapse is needed to appropriately direct those patients to HCT when necessary, while avoiding HCT in others. High disease burden prior to CAR-T infusion, loss of B-cell aplasia and detection of measurable residual disease by flow cytometry or next-generation sequencing following CAR-T therapy associate with a higher relapse risk and may identify patients requiring consolidative HCT for relapse prevention. SUMMARY: There is a pressing need to determine when CD19 CAR-T alone is likely to be curative and when a consolidative HCT will be required. We discuss the current state of knowledge and future directions.

Development of T-cell immunotherapy for hematopoietic stem cell transplantation recipients at risk of leukemia relapse.

Leukemia relapse remains the major cause of allogeneic hematopoietic stem cell transplantation (HCT) failure, and the prognosis for patients with post-HCT relapse is poor. There is compelling evidence that potent selective antileukemic effects can be delivered by donor T cells specific for particular minor histocompatibility (H) antigens. Thus, T-cell receptors (TCRs) isolated from minor H antigen-specific T cells represent an untapped resource for developing targeted T-cell immunotherapy to manage post-HCT leukemic relapse. Recognizing that several elements may be crucial to the efficacy and safety of engineered T-cell immunotherapy, we developed a therapeutic transgene with 4 components: (1) a TCR specific for the hematopoietic-restricted, leukemia-associated minor H antigen, HA-1; (2) a CD8 coreceptor to promote function of the class I-restricted TCR in CD4+ T cells; (3) an inducible caspase 9 safety switch to enable elimination of the HA-1 TCR T cells in case of toxicity; and (4) a CD34-CD20 epitope to facilitate selection of the engineered cell product and tracking of transferred HA-1 TCR T cells. The T-cell product includes HA-1 TCR CD4+ T cells to augment the persistence and function of the HA-1 TCR CD8+ T cells and includes only memory T cells; naive T cells are excluded to limit the potential for alloreactivity mediated by native TCR coexpressed by HA-1 TCR T cells. We describe the development of this unique immunotherapy and demonstrate functional responses to primary leukemia by CD4+ and CD8+ T cells transduced with a lentiviral vector incorporating the HA-1 TCR transgene construct.

Intent-to-treat leukemia remission by CD19 CAR T cells of defined formulation and dose in children and young adults.

Transitioning CD19-directed chimeric antigen receptor (CAR) T cells from early-phase trials in relapsed patients to a viable therapeutic approach with predictable efficacy and low toxicity for broad application among patients with high unmet need is currently complicated by product heterogeneity resulting from transduction of undefined T-cell mixtures, variability of transgene expression, and terminal differentiation of cells at the end of culture. A phase 1 trial of 45 children and young adults with relapsed or refractory B-lineage acute lymphoblastic leukemia was conducted using a CD19 CAR product of defined CD4/CD8 composition, uniform CAR expression, and limited effector differentiation. Products meeting all defined specifications occurred in 93% of enrolled patients. The maximum tolerated dose was 106 CAR T cells per kg, and there were no deaths or instances of cerebral edema attributable to product toxicity. The overall intent-to-treat minimal residual disease-negative (MRD-) remission rate for this phase 1 study was 89%. The MRD- remission rate was 93% in patients who received a CAR T-cell product and 100% in the subset of patients who received fludarabine and cyclophosphamide lymphodepletion. Twenty-three percent of patients developed reversible severe cytokine release syndrome and/or reversible severe neurotoxicity. These data demonstrate that manufacturing a defined-composition CD19 CAR T cell identifies an optimal cell dose with highly potent antitumor activity and a tolerable adverse effect profile in a cohort of patients with an otherwise poor prognosis. This trial was registered at www.clinicaltrials.gov as #NCT02028455.

Cytomegalovirus-specific T cells are primed early after cord blood transplant but fail to control virus in vivo.

A disadvantage of umbilical cord blood transplantation (UCBT) is the delay in immune reconstitution, placing patients at increased risk for infections after transplant. Cytomegalovirus (CMV) in particular has been shown to cause significant morbidity in patients undergoing UCBT. Here, we comprehensively evaluate the development of CD4(+) and CD8(+) T-cell responses to CMV in a cohort of patients that underwent double UCBT. Our findings demonstrate conclusively that a diverse polyclonal CMV-specific T-cell response derived from the UCB graft is primed to viral antigens as early as day 42 after UCBT, but these T cells fail to achieve sufficient numbers in vivo to control CMV reactivations. This is not due to an inherent inability of UCB-derived T cells to proliferate, as these T cells underwent rapid proliferation in vitro. The TCR diversity and antigen specificity of CMV-specific T cells remained remarkably stable in the first year after transplant, suggesting that later control of virus replication results from improved function of T cells primed early after transplant and not from de novo responses derived from later thymic emigrants. Ex vivo expansion and adoptive transfer of CMV-specific T cells isolated from UCBT recipients early after transplant could augment immunity to CMV.

Engineering human peripheral blood stem cell grafts that are depleted of naïve T cells and retain functional pathogen-specific memory T cells.

Graft-versus-host disease (GVHD) is a frequent major complication of allogeneic hematopoietic cell transplantation (HCT). Approaches that selectively deplete T cells that cause GVHD from allogeneic stem cell grafts and preserve T cells specific for pathogens may improve HCT outcomes. It has been hypothesized that the majority of T cells that can cause GVHD reside within the naïve T cell (TN) subset, and previous studies performed in mouse models and with human cells in vitro support this hypothesis. As a prelude to translating these findings to the clinic, we developed and evaluated a novel 2-step clinically compliant procedure for manipulating peripheral blood stem cells (PBSC) to remove TN, preserve CD34(+) hematopoietic stem cells, and provide for a fixed dose of memory T cells (TM) that includes T cells with specificity for common opportunistic pathogens encountered after HCT. Our studies demonstrate effective and reproducible performance of the immunomagnetic cell selection procedure for depleting TN. Moreover, after cell processing, the CD45RA-depleted PBSC products are enriched for CD4(+) and CD8(+) TM with a central memory phenotype and contain TM cells that are capable of proliferating and producing effector cytokines in response to opportunistic pathogens.

Cyclin-A1 represents a new immunogenic targetable antigen expressed in acute myeloid leukemia stem cells with characteristics of a cancer-testis antigen.

Targeted T-cell therapy is a potentially less toxic strategy than allogeneic stem cell transplantation for providing a cytotoxic antileukemic response to eliminate leukemic stem cells (LSCs) in acute myeloid leukemia (AML). However, this strategy requires identification of leukemia-associated antigens that are immunogenic and exhibit selective high expression in AML LSCs. Using microarray expression analysis of LSCs, hematopoietic cell subpopulations, and peripheral tissues to screen for candidate antigens, cyclin-A1 was identified as a candidate gene. Cyclin-A1 promotes cell proliferation and survival, has been shown to be leukemogenic in mice, is detected in LSCs of more than 50% of AML patients, and is minimally expressed in normal tissues with exception of testis. Using dendritic cells pulsed with a cyclin-A1 peptide library, we generated T cells against several cyclin-A1 oligopeptides. Two HLA A*0201-restricted epitopes were further characterized, and specific CD8 T-cell clones recognized both peptide-pulsed target cells and the HLA A*0201-positive AML line THP-1, which expresses cyclin-A1. Furthermore, cyclin-A1-specific CD8 T cells lysed primary AML cells. Thus, cyclin-A1 is the first prototypic leukemia-testis-antigen to be expressed in AML LSCs. The pro-oncogenic activity, high expression levels, and multitude of immunogenic epitopes make it a viable target for pursuing T cell-based therapy approaches.

qPCR assay for detection of Woodchuck Hepatitis Virus Post-Transcriptional Regulatory Elements from CAR-T and TCR-T cells in fresh and formalin-fixed tissue.

As adoptive cellular therapies become more commonplace in cancer care, there is a growing need to monitor site-specific localization of engineered cells-such as chimeric antigen receptor T (CAR-T) cells and T-cell receptor T (TCR-T) cells-in patients' tissues to understand treatment effectiveness as well as associated adverse events. Manufacturing CAR-T and TCR-T cells involves transduction with viral vectors commonly containing the WPRE gene sequence to enhance gene expression, providing a viable assay target unique to these engineered cells. Quantitative PCR (qPCR) is currently used clinically in fresh patient tissue samples and blood with target sequences specific to each immunotherapy product. Herein, we developed a WPRE-targeted qPCR assay that is broadly applicable for detection of engineered cell products in both fresh and archival formalin-fixed paraffin embedded (FFPE) tissues. Using both traditional PCR and SYBR Green PCR protocols, we demonstrate the use of this WPRE-targeted assay to successfully detect two CAR-T cell and two TCR-T cell products in FFPE tissue. Standard curve analysis reported a reproducible limit of detection at 100 WPRE copies per 20µL PCR reaction. This novel and inexpensive technique could provide better understanding of tissue abundance of engineered therapeutic T cells in both tumor and second-site toxicity tissues and provide quantitative assessment of immune effector cell trafficking in archival tissue.

Discovery of U2AF1 neoantigens in myeloid neoplasms.

BACKGROUND: Myelodysplastic syndromes (MDS) arise from somatic mutations acquired in hematopoietic stem and progenitor cells, causing cytopenias and predisposing to transformation into secondary acute myeloid leukemia (sAML). Recurrent mutations in spliceosome genes, including U2AF1, are attractive therapeutic targets as they are prevalent in MDS and sAML, arise early in neoplastic cells, and are generally absent from normal cells, including normal hematopoietic cells. MDS and sAML are susceptible to T cell-mediated killing, and thus engineered T-cell immunotherapies hold promise for their treatment. We hypothesized that targeting spliceosome mutation-derived neoantigens with transgenic T-cell receptor (TCR) T cells would selectively eradicate malignant cells in MDS and sAML. METHODS: We identified candidate neoantigen epitopes from recurrent protein-coding mutations in the spliceosome genes SRSF2 and U2AF1 using a multistep in silico process. Candidate epitopes predicted to bind human leukocyte antigen (HLA) class I, be processed and presented from the parent protein, and not to be subject to tolerance then underwent in vitro immunogenicity screening. CD8+ T cells recognizing immunogenic neoantigen epitopes were evaluated in in vitro assays to assess functional avidity, confirm the predicted HLA restriction, the potential for recognition of similar peptides, and the ability to kill neoplastic cells in an antigen-specific manner. Neoantigen-specific TCR were sequenced, cloned into lentiviral vectors, and transduced into third-party T cells after knock-out of endogenous TCR, then tested in vitro for specificity and ability to kill neoplastic myeloid cells presenting the neoantigen. The efficacy of neoantigen-specific T cells was evaluated in vivo in a murine cell line-derived xenograft model. RESULTS: We identified two neoantigens created from a recurrent mutation in U2AF1, isolated CD8+ T cells specific for the neoantigens, and demonstrated that transferring their TCR to third-party CD8+ T cells is feasible and confers specificity for the U2AF1 neoantigens. Finally, we showed that these neoantigen-specific TCR-T cells do not recognize normal hematopoietic cells but efficiently kill malignant myeloid cells bearing the specific U2AF1 mutation, including primary cells, in vitro and in vivo. CONCLUSIONS: These data serve as proof-of-concept for developing precision medicine approaches that use neoantigen-directed T-cell receptor-transduced T cells to treat MDS and sAML.

NY-ESO-1 is a ubiquitous immunotherapeutic target antigen for patients with myxoid/round cell liposarcoma.

BACKGROUND: Myxoid/round cell liposarcoma (MRCL) is the second most common liposarcoma subtype, accounting for >33% of liposarcomas and approximately 10% of all soft tissue sarcomas. Although MRCL is a chemosensitive subtype, patients with metastatic disease have a poor outcome. NY-ESO-1 is a cancer-testis antigen (also known as cancer germ cell antigen) that has been successfully targeted in vaccine trials and in adoptive T-cell therapy trials for the treatment of several solid tumors. METHODS: The authors investigated the feasibility of targeting NY-ESO-1 in patients with MRCL by evaluating the prevalence of NY-ESO-1 expression in tumors using immunohistochemistry and quantitative reverse transcriptase-polymerase chain reaction analysis. NY-ESO-1-specific tumor recognition by NY-ESO-1-specific T-cells also was analyzed using a chromium release assay. RESULTS: A search of the University of Washington Sarcoma Tissue Bank identified paraffin-embedded tumor samples from 25 patients with MRCL. NY-ESO-1 expression was observed in every MRCL tumor assessed (100%); in 18 tumors (72%), staining was homogenous. In all but 2 tumors, staining was sufficiently robust (2+) that such patients would be eligible for clinical trials of NY-ESO-1-directed therapy. By using NY-ESO-1 specific, CD8-positive T-cells, the in vitro sensitivity of myxoid liposarcoma cell lines to antigen-specific lysis was demonstrated. CONCLUSIONS: The current results establish NY-ESO-1 as an important target antigen for the treatment of patients with MRCL.

Leukemia-associated minor histocompatibility antigen discovery using T-cell clones isolated by in vitro stimulation of naive CD8+ T cells.

T-cell immunotherapy that targets minor histocompatibility (H) antigens presented selectively by recipient hematopoietic cells, including leukemia, could prevent and treat leukemic relapse after hematopoietic cell transplantation without causing graft-versus-host disease. To provide immunotherapy that can be applied to a majority of transplantation recipients, it is necessary to identify leukemia-associated minor H antigens that result from gene polymorphisms that are balanced in the population and presented by common human leukocyte antigen alleles. Current approaches for deriving minor H antigen-specific T cells, which provide essential reagents for the molecular identification and characterization of the polymorphic genes that encode the antigens, rely on in vivo priming and are often unsuccessful. We show that minor H antigen-specific cytotoxic T lymphocyte precursors are found predominantly in the naive CD8(+) T-cell subset and provide an efficient strategy for in vitro priming of native T cells to generate T cells to a broad diversity of minor H antigens presented with common human leukocyte antigen alleles. We used this approach to derive a panel of stable cytotoxic T lymphocyte clones for discovery of genes that encode minor H antigens and identify a novel antigen expressed on acute myeloid leukemia stem cells and minimally in graft-versus-host disease target tissues.

The B-cell tumor-associated antigen ROR1 can be targeted with T cells modified to express a ROR1-specific chimeric antigen receptor.

Monoclonal antibodies and T cells modified to express chimeric antigen receptors specific for B-cell lineage surface molecules such as CD20 exert antitumor activity in B-cell malignancies, but deplete normal B cells. The receptor tyrosine kinase-like orphan receptor 1 (ROR1) was identified as a highly expressed gene in B-cell chronic lymphocytic leukemia (B-CLL), but not normal B cells, suggesting it may serve as a tumor-specific target for therapy. We analyzed ROR1-expression in normal nonhematopoietic and hematopoietic cells including B-cell precursors, and in hematopoietic malignancies. ROR1 has characteristics of an oncofetal gene and is expressed in undifferentiated embryonic stem cells, B-CLL and mantle cell lymphoma, but not in major adult tissues apart from low levels in adipose tissue and at an early stage of B-cell development. We constructed a ROR1-specific chimeric antigen receptor that when expressed in T cells from healthy donors or CLL patients conferred specific recognition of primary B-CLL and mantle cell lymphoma, including rare drug effluxing chemotherapy resistant tumor cells that have been implicated in maintaining the malignancy, but not mature normal B cells. T-cell therapies targeting ROR1 may be effective in B-CLL and other ROR1-positive tumors. However, the expression of ROR1 on some normal tissues suggests the potential for toxi-city to subsets of normal cells.

Utility of the Treatment-Related Mortality (TRM) score to predict outcomes of adults with acute myeloid leukemia undergoing allogeneic hematopoietic cell transplantation.

There is long-standing interest in estimating non-relapse mortality (NRM) after allogeneic hematopoietic cell transplantation (HCT) for AML, but existing tools have limited discriminative capacity. Using single-institution data from 861 adults with AML, we retrospectively examined the Treatment-Related Mortality (TRM) score, originally developed to predict early mortality following induction chemotherapy, as a predictor of post-HCT outcome. NRM risks increased stepwise across the four TRM score quartiles (at 3 years: 9% [95% confidence interval: 5-13%] in Q1 vs. 28% [22-34%] in Q4). The 3-year risk of relapse was lower in patients with lower TRM score (26% [20-32%] in Q1 vs. 37% [30-43%] in Q4). Consequently, relapse-free survival (RFS) and overall survival (OS) estimates progressively decreased (RFS at 3 years: 66% [59-72%] in Q1 vs. 36% [29-42%] in Q4; OS at 3 years: 72% [66-78%] in Q1 vs. 39% [33-46%] in Q4). With a C-statistic of 0.661 (continuous variable) or 0.642 (categorized by quartile), the TRM score predicted NRM better than the Pretransplantation Assessment of Mortality (PAM) score (0.603) or the HCT-CI/age composite score (0.576). While post-HCT outcome prediction remains challenging, these findings suggest that the TRM score may be useful for risk stratification for adults with AML undergoing allogeneic HCT.

Hematopoietic Cell Transplantation in the Treatment of Pediatric Acute Myelogenous Leukemia and Myelodysplastic Syndromes: Guidelines from the American Society of Transplantation and Cellular Therapy.

The role of allogeneic hematopoietic stem cell transplantation (HCT) in the treatment of acute myelogenous leukemia (AML) in children is reviewed and critically evaluated in this evidence-based review. Specific criteria were used for searching the published literature, grading the quality and strength of evidence, and assigning the strength of treatment recommendations. Genomic characterization and response to therapy have been critical in the risk stratification of pediatric AML. Although some children are cured with chemotherapy alone, allogeneic HCT offers a survival benefit in selected patients with certain unfavorable risk features and is the standard of care for children who relapse following initial treatment with chemotherapy. Important aspects of HCT include recipient characteristics, donor source, and preparative regimen. The goals of HCT are to reduce incidence of relapse, enhance graft-versus-leukemia (GVL) effects, and minimize graft-versus-host disease. Relapse following HCT remains a significant cause of treatment failure, and interventions pre- and post-HCT, especially those that may augment GVL, are an important focus of ongoing investigations.

Hematopoietic Cell Transplantation after CD19 Chimeric Antigen Receptor T Cell-Induced Acute Lymphoblastic Lymphoma Remission Confers a Leukemia-Free Survival Advantage.

Consolidative hematopoietic cell transplantation (HCT) after CD19 chimeric antigen receptor (CAR) T cell therapy is frequently performed for patients with refractory/ relapsed B cell acute lymphoblastic leukemia (B-ALL). However, there is controversy regarding the role of HCT following remission attainment. We evaluated the effect of consolidative HCT on leukemia-free survival (LFS) in pediatric and young adult subjects following CD19 CAR T cell induced remission. We evaluated the effect of consolidative HCT on LFS in pediatric and young adult subjects treated with a 41BB-CD19 CAR T cell product on a phase 1/2 trial, Pediatric and Young Adult Leukemia Adoptive Therapy (PLAT)-02 (ClinicalTrials.gov identifier NCT02028455), using a time-dependent Cox proportional hazards statistical model. Fifty of 64 subjects enrolled in PLAT-02 phase 1 and early phase 2 were evaluated, excluding 14 subjects who did not achieve remission, relapsed, or died before day 63 post-CAR T cell therapy. An improved LFS (P = .01) was observed in subjects who underwent consolidative HCT after CAR T cell therapy versus watchful waiting. Consolidative HCT improved LFS specifically in subjects who had no prior history of HCT, with a trend toward significance (P = .09). This benefit was not evident when restricted to the cohort of 34 subjects with a history of prior HCT (P = .45). However, for subjects who had CAR T cell functional persistence of 63 days or less, inclusive of those with a history of prior HCT, HCT significantly improved LFS outcomes (P = .01). These data support the use of consolidative HCT following CD19 CAR T cell-induced remission for patients with no prior history of HCT and those with short functional CAR T cell persistence.